Probing hyperconjugation experimentally with the conformational deuterium isotope effect

Area of Science:

• Chemistry
• Physical Chemistry
• Spectroscopy

Background:

• Hyperconjugation is crucial in chemistry, particularly the anomeric effect in oxygen systems.
• Hyperconjugative effects in second- and third-row chalcogens (sulfur, selenium) are less understood and debated.
• Conformational equilibria influence molecular properties and reactivity.

Purpose of the Study:

• To experimentally probe conformational equilibria in dioxane, dithiane, and diselenane analogues.
• To investigate the periodic trend of hyperconjugation across chalcogens.
• To quantify the deuterium isotope effect in these systems.

Main Methods:

• Variable-temperature, dynamic Nuclear Magnetic Resonance (NMR) spectroscopy.
• Saunders' isotopic perturbation method.
• Infrared (IR) spectroscopy and Density Functional Theory (DFT) calculations.

Main Results:

• The conformational deuterium isotope effect was measured as 252.1 J/mol for oxygen, 28.3 J/mol for sulfur, and 7.1 J/mol for selenium analogues.
• A decreasing trend in hyperconjugation (n(x)→σ(C-H(D)) interaction) was observed down the chalcogen group.
• Results were supported by IR spectroscopy, DFT calculations, and natural bond order analysis.

Conclusions:

• The study provides a sensitive method to probe conformational equilibria using deuterium isotope effects.
• A clear periodic trend in hyperconjugation across chalcogens has been established.
• The findings clarify the nature and strength of hyperconjugative interactions in sulfur and selenium compounds.